Abstract
Shell operates a lower olefins unit at its petrochemical refinery in Wesseling, Germany. The unit was built in the 1970ies and required debottlenecking to increase its capacity. For this it was necessary to check and upgrade the process safety equipment. The key items listed below illustrate the magnitude of the project:More than 30 new heat exchangers due to a higher duty requirement and partly due to vibration issuesMore than 400 new sensorsA two-digit number of new safety instrumented functions (tripping functions)More than 40 re-designed relief valvesA new flare line DN600 about 100 m longDuring the execution phase of the project more than 1000 craftsmen and more than 10 cranes were in active in the fieldThree topics will be discussed here:New relief valves and a new flare line to deal with the flare backpressure issue during a total power failureThe redesign of a heat exchanger, which was subject to vibrationThe process safety improvement to protect against the runaway in a selective hydrogenation reactorThe selective hydrogenation reactor is adiabatic and the reactions (hydrogenation of Acetylene) are exothermic. Low flow through the reactor leads to temperature increase which in turn starts secondary reactions that generate excessive heat. In confined sections of the reactor bed, this could cause significant temperature excursions and further reactions (polymerization). These localised temperature excursions, so-called hot-spots, are a threat, especially if the formation occurs close to the reactor walls. The new safety approach included the installation of 8 temperature sensors per reactor-bed and a new safeguarding concept with emergency depressurization.
However, starting up the newly equipped reactor with fresh catalyst was a challenge.
